When Synchronous/Timing belts are used on pulleys with Vertical Shafts (i.e., the belt is riding on its side), gravity tends to pull the belt downward, so vertical shaft systems should have at least one pulley with flanges on both sides (normally the smaller pulley), and the remaining pulley (normally the larger pulley) should be flanged at least on the bottom side.
The general guideline provided by manufacturers is that in all synchronous belt drive systems, at least one pulley should have flanges on both sides of the pulley. (Normally the smaller pulley is flanged on both sides.)
When the center distance between shafts, is eight times or more the diameter of the smaller pulley, both pulleys should have flanges. Normally pulleys are flanged on both sides up to 80 tooth (Standard Pulley) and over 90 tooth pulleys are not flanged.
Pictured: Large ACHE Pulley w/bolted-on flanges. Pictured: Pulley with rolled flanges.